As well known, a shape memory alloy, such as a Ti—Ni alloy, exhibits a remarkable shape memory effect in association with martensitic reverse transformation and exhibits spontaneous shape recovery and excellent spring characteristics (superelasticity) in a parent phase region after the reverse transformation from a martensite region. The superelasticity is observed in a number of shape memory alloys and, among others, is particularly remarkable in the Ti—Ni alloy and a Ti—Ni—X alloy (X=V, Cr, Co, Nb, or the like).
The shape memory effect of the Ti—Ni alloy is described in Patent Document 1. The superelasticity of the Ti—Ni alloy is described in Patent Document 2.
On the other hand, the shape memory effect and the superelasticity of the Ti—Ni—X alloy are described, for example, in Patent Documents 3 and 4 for a Ti—Ni—V alloy and in Patent Document 5 for a Ti—Ni—Nb alloy.
As compared with the Ti—Ni alloy, the Ti—Ni—Nb alloy used in this invention exhibits a characteristic that temperature hysteresis of stresses is increased by applying a stress. Therefore, the Ti—Ni—Nb alloy is put into practical use as a joint for reactor piping.
Stent treatment is a new technique rapidly put into use in recent years. The stent is a mesh-like metal tube to be placed in a living body in order to prevent renarrowing or restenosis of a narrow portion, such as a blood vessel, after it is expanded. The stent is reduced in diameter and received in an end portion of a catheter. After introduced into the narrow portion, the stent is released from the catheter and expanded to be attached to an inner wall of a lumen such as a blood vessel.
In case of PTCA (percutaneous transluminal coronary angioplasty), the stent is expanded following a blood vessel expanding operation by inflation of a balloon set on an inner wall for housing. The stent is called a balloon expandable stent and formed by the use of a metal such as stainless steel or tantalum.
On the other hand, in order to prevent rupture of an aneurysm which may result in a subarachnoid hemorrhage or the like, blood supply to the aneurysm is stopped. As one of such techniques, use is made of embolization in which a metal coil, such as a platinum coil, is implanted into the aneurysm so as to form a blood clot. However, it is pointed out that a part of the blood clot may possibly be released from the metal and carried by a bloodstream to a periphery to block a blood vessel. In order to avoid this, consideration is made about a covered stent technique in which the aneurysm is embolized by the use of a graft. In this case, simultaneously when the stent is released from the catheter, the stent is expanded by its own spring function to press the graft against a blood vessel wall. Such stent is called a self expandable stent. For the self expandable stent, a material having an excellent spring characteristic is desired.
The Ti—Ni shape memory alloy is characterized in that, at a temperature above a reverse transformation finish temperature (Af point) at which reverse transformation of the alloy starting from a reverse transformation start temperature (As point) is finished, the alloy which has been deformed under an external load is recovered into an original shape simultaneously when the external load is released and that recoverable deformation reaches about 7% in case of an elongation strain. Herein, the As point means a shape recovery start temperature while the Af point means a shape recovery finish temperature (shape recovery temperature). For use as the stent, a hoop-shaped stent is formed into a size slightly greater than the lumen where the stent is to be placed. The stent is reduced in diameter and mounted to the catheter. Simultaneously when the stent is released from the catheter, the stent is spontaneously recovered into its original diameter to be brought into tight contact with the lumen. Thus, the alloy has the Af point not lower than a living body temperature (around 37° C.). As well as the above-mentioned merits, such superelastic stent has several demerits, such as occurrence of damage in the blood vessel wall, a positioning error in placement, lack in deliverability, and so on due to its spontaneous shape recovery characteristic. Therefore, it is difficult to use the superelastic stent in a blood vessel system such as a coronary system.
The stent for PTCA is preferably made of a metal material having a low elastic limit, which hardly damages the blood vessel and is excellent in deliverability. However, there is left a problem that a pressing force (expanding force) against a lumen wall after expansion is weak. As means to solve the problem, a stent using a shape memory alloy is proposed. Patent Document 6 describes that a Ti—Ni—Nb alloy, which is a material similar to that used in this invention, is applied to a stent. This document describes that the stent made of a Ti—Ni—Nb shape memory alloy and having a low Young's modulus upon shape recovery and a high Young's modulus upon shape deformation under an external load is obtained when the ratio of stress on loading to the stress on unloading at the respective inflection points on a stress-strain curve in alloy deformation is at least about 2.5:1. This stent exhibits superelasticity at the living body temperature after it is released from the catheter but does not sufficiently solve the above-mentioned problem (arbitrariness in positioning) as required in PTCA.
In Patent Document 7, the present inventors have proposed a stent closely related to this invention. Specifically, proposal is made of the stent which exhibits no shape memory at the living body temperature during insertion into the living body and exhibits superelasticity after shape recovery by inflation of a balloon. In the embodiment, it is described that the stent made of a Ti—Ni alloy or a Ti—Ni—X alloy (X=Cr, V, Cu, Fe, Co, or the like) is subjected to strong deformation to thereby elevate a recovery temperature. However, this document does not refer to a graded function of the stent. Further, a strain is given only by strong deformation of a slotted stent received in the catheter. Depending upon a slot shape, a sufficient effect is not obtained. Patent Document 8 discloses a stent using a Ti—Ni alloy or a Ti—Ni—X alloy and proposes to partly change the stiffness of the material by heat treatment. Specifically, such change by heat treatment provides a series of superelastic portions having a relatively high stiffness and plastically deformable portions having a relatively low stiffness (in the description, portions where the superelasticity is destroyed) which are alternately arranged. Thus, this technique is different in gist and means intended by this invention.
The stent is required to have functions such as deliverability (accessibility to peripheral or distal parts), prevention of restenosis (strong expanding force after placement), and flexible shape conformability. Following recent increase in stent treatment cases, a problem of restenosis after placement of the stent is exposed.
For example, in case where the stent is placed at a tortuous coronary artery lesion, a restenotic lesion after placement tends to occur at opposite ends of the stent which are most susceptible to stimulation. In this event, re-placement of the stent or bypass surgery must be performed so that mental and physical burdens on a patient are extremely heavy. The deliverability (accessibility to peripheral parts) during operation is achieved by using a stainless steel material. The strong expanding force after placement is achieved by a conventional Ti—Ni—X alloy superelastic material. In order to achieve the flexible shape conformability (relaxation of stimulation to the restenotic lesion) after placement, it is proposed to weaken the expanding force of the stent to relax the stimulation to the lumen. However, this results in loss of an inherent function (reinforcement of the lumen) of the stent. Practically, a material-based approach is difficult. Therefore, at present, stent processing such as designing of a slot shape is relied upon as secondary means. However, various essential problems are left unsolved.
Patent Document 1: U.S. Pat. No. 3,174,851
Patent Document 2: JP S58-161753 A
Patent Document 3: JP S63-171844 A
Patent Document 4: JP S63-14834 A
Patent Document 5: U.S. Pat. No. 4,770,725
Patent Document 6: JP H11-42283 A
Patent Document 7: JP H11-99207 A
Patent Document 8: JP 2003-505194 A